Posts Tagged ‘rope’

We’ve been talking about pulleys for awhile now, and last week we introduced the term friction coefficient, numerical values derived during testing which quantify the amount of friction present when different materials interact. Friction coefficients for common materials are routinely presented in engineering texts like Marks’ Standard Handbook for Mechanical Engineers. But there are circumstances when more specificity is required, such as when the U.S. Navy, more specifically the Navy Material Command, tested the interaction between various synthetic ropes and ship capstans and developed their own specialized friction coefficients in the process.

Navy Capstans and the Development of Specialized Friction Coefficients

Capstansare similar to pulleys but have one key difference, they’re made so rope can be wound around them multiple times. When the Navy set out to determine which synthetic rope worked best with their capstans, they did testing and developed highly specialized friction coefficients in the process. This research was at one time Top Secret but has now been declassified. To read more about it, follow this link to the actual handbook:

Sometimes the simplest alteration in design results in a huge improvement, a truth I’ve discovered more than a few times during my years as an engineering expert. Last time we introduced the simple pulleyand revealed that its usefulness was limited to the strength of the pulling force behind it. Hundreds of years ago that force was most often supplied by a man and his biceps. But ancient Greeks found an ingenious and simple way around this limitation, which we’ll highlight today by way of a modern design engineer’s tool, the free body diagram.

Around 400 BC, the Greeks noticed that if they detached the simple pulley from the beam it was affixed to in our last blog and instead allowed it to be suspended in space with one of its rope ends fastened to a beam, the other rope end to a pulling force, something interesting happened.

The Simple Pulley Improved

It was much easier to lift objects while suspended in air. As a matter of fact, it took 50% less effort. To understand why, let’s examine what engineers call a free body diagram of the pulley in our application, as shown in the blue inset box and in greater detail below.

Using a Free Body Diagram to Understand Simple Pulleys

The blue insert box in the first illustration highlights the subject at hand. A free body diagram helps engineers analyze forces acting upon a stationary object suspended in space. The forces acting upon the object, in our case a simple pulley, represent both positive and negative values. The free body diagramabove indicates that forces pointing up are, by engineering convention, considered to be positive, while downward forces are negative. The basic rule of all free body diagrams is that in order for an object to remain suspended in a fixed position in space, the sum of all forces acting upon it must equal zero.

We’ll see how the free body diagram concept is instrumental in understanding the improvement upon the action of a simple pulley next time, when we attack the math behind it.